The reason for Zonal Pelargoniums (bedding geranium) leaves turning red is that the temperature around them is too low. It is nothing to do with watering unless the plants are very dry at the roots - not likely as you are tending to them.

They will return to green when the temperature comes back to their liking.

Agree with that bigolob. I only added the water bit as a separate point and not for the cause,of leaves reddening. I have seen so many pelargoniums ruined by overwatering in the garden.....pelargoniums need very little extra watering if outside